URIC ACID ( from Gk. olpop, ouron, urine), A compound of carbon, hydrogen, nitrogen, and oxygen, which forms a loose white powder or scales consisting of minute crystals de void of smell or taste, only very slightly soluble in water (1 part requiring about 15.000 parts of cold and 1000 of boiling water), and quite in soluble in alcohol and ether. It is soluble with out decomposition in strong sulphuric acid, and it may be thrown down from this solution by the addition of water. It is also soluble in the carbonates, horates, phosphates, lactates, and acetates of the alkalies, extracting from these salts a part of their base, with which it forms acid urates. Litmus paper is reddened by its moist crystals, or by a hot aqueous solution. The acid is not volatile, and by dry distillation is decomposed into carbonate of ammonia, urea, eyanuric acid, hydrocyanic acid, etc. Uric acid acts as a very weak dibasic add, forming with bases two series of salts, the neutral and the acid, of which the former are the more soluble. Among the most important are the acid urates of sodium, potassium, lithium, and ammonium. The urate of lithium is more soluble than any other urate, and hence lithia water is an important therapeutic agent in converting uric acid and the more insoluble urates into a soluble salt in the living body. The urates are frequently called llithates,' and time acid itself 'Mimic acid.' Uric acid is widely distributed throughout the animal organism. It occurs not only in the urine of man and carnivorous animals, but is the chief constituent (either free or in combination) of many calculi in the kidneys or bladder, and of numerous urinary sediments. The urinary secretion of birds and reptiles consists almost entirely of urates, which are also found in the excrements of caterpillars, butterflies, beetles, etc., and of many mollusks. Moreover, in very minute quantities, it occurs as a urate in healthy blood. in which fluid it has been found in excess in gout and Bright's disease, and is a constituent of the aqueous extract of the spleen, liver, lungs, pancreas, and brain. The chalk-stones occurring about the smaller joints and in the lobes of the ear of gouty patients consist mainly of mate of soda.
The best and most ready mode of obtaining uric acid on a large scale is from guano, or from the excrements of snakes or fowls, which consist largely of mates. For this purpose the mass is boiled with potash, which expels any ammonia that is present, and a stream of carbonic acid is then passed through the strained potash solution, which throws down 'acid urate of potash. This precipitate is dissolved in water, and decom posed by hydrochloric acid, which throws down the uric acid in minute crystals. The form in which this acid crystallizes is liable to great variations. Sometimes we have flat tablets re sembling whetstones, or sections with a double knife through biconvex lenses; at other times, more or less perfect rhombic tablets, and some times hexagonal plates, barrel-shaped prisms, and toothed crystals. it is, however, difficult to
identify uric acid crystals by direct inspection; so that, to prove the presence of uric acid, tests of a chemical nature must be applied. The so called murexid reaction constitutes an excellent test: the substance submitted for examination is treated with a little strong nitric acid, and the mixture is evaporated to dryness at a gentle heat ; the residue is moistened with a trace of ammonia, which, if uric acid was present in the original substance, produces a brilliant purple coloration ; a drop of caustic soda changes the color to a reddish blue that disappears on warm ing the solution. The test is based on the fact that warming with nitric acid converts urea into purpurie acid, the acid ammonium salt of which—called murexid—has a brilliant reddish purple color.
By the oxidation of uric acid two series of in teresting products are obtained. If nitric acid is used as the oxidizing agent, uric acid splits up into urea and alloxan (a ureide). If, instead of nitric acid, potassium permanganate is em ployed, uric acid breaks up into carbonic acid and allantoin (another ureide). Alloxan can be readily transformed into parabanic acid (the ureide of oxalic acid), and from the latter urea and oxalic acid can be obtained with equal readi ness. On the other hand, allantoin can be con verted, by a series of simple reactions, into urea, allanturic acid (glyoxyl urea), parabanie acid (oxalyl urea), afid hydantoie 'acid. the latter being closely allied to hydantoin (glyeolyl urea). The substituted ureas (ureides) obtained from uric acid may also he prepared directly from urea. (See UREAS. COMPOUND.) It is, there fore, clear that uric acid and urea are chemi cally closely related to each other. In mammals nitrogenous substances are disintegrated mainly into urea; in birds the main product of disin tegration is uric acid. The formation of both urea and uric acid takes place most probably in the liver. The urine of mammals sometimes con tains allantoin, hydrated parabanic acid, oxalic acid. and other products, which are obtained in the laboratory by oxidizing uric acid.
The products of decomposition of uric acid were first thoroughly investigated, in a series of celebrated researches, by Liebig and Wohler. Then the chemical relati9nship of urea and uric acid was clearly established. It has since been shown that uric acid is a di-ureide, i.e. a sub stitution product containing two molecules of urea. After much painstaking research, carried on by some of the best chemists of the century, it was possible to demonstrate the true chemical constitution of uric acid, and finally Horbac zewsky succeeded in preparing this important organic substance by a synthetic method; Beh rend and Roosen subsequently discovered an other synthesis of uric acid, which is relatively simple and perfectly well understood in the light of our knowledge of the constitution of uric acid. The chemical constitution of uric acid is rep resented by the following structural formula, first proposed by :Medici's: